Donna is one year older than her sister, and her sister is 3 years older than their brother. If the sum of their ages is 31, how old is Donna?

Answer 1

B + B + 3 + B + 4 = 31

Solve for B.

Answer 2

D = s + 1

s = b + 3 ... b = s - 3

D + s + b = 31

s + 1 + s + s - 3 = 31

find s, then substitute back